Current wireless local area network (WLAN) systems such as IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ad support multiple transmit antennas at both the transmitter (Tx) and the receiver (Rx). Multiple transmit and receive antennas are supported in access points (APs), wireless transmit/receive units (WTRU), and other non-WTRU devices. For example, 802.11n supports up to 4 transmit and 4 receive antennas, while 802.11ac supports up to 8 transmit and 8 receive antennas. For such systems, transmit beamforming (also referred to as “precoding”) may be used to improve the coverage area and/or throughput.
For transmit beamforming the transmitter may need to acquire channel state information (CSI) of the channel (e.g. amplitude/phase) prior to transmission, using channel reciprocity and/or CSI feedback. The actual transmit beamforming coefficients are then derived as a function of the channel state information. The more accurate the transmitter-side CSI is, the more beneficial transmit beamforming may be.
It is anticipated that a requirement of future WLAN specifications may be higher spectral efficiency than is currently available. Also, systems which support a large number of meter type control (MTC) devices may require an improvement in BSS capacity. Several general issues may be hindrances to achieving higher spectral efficiencies and capacity. Current beamforming report methods lead to significant feedback overhead, thereby reducing spectral efficiency and BSS capacity. Therefore there is a need for improved methods for beamforming, feedback, and sounding.